The present invention relates to an improved process for preparing fluorine by electrolysis of calcium fluoride (CaF.sub.2), and, more particularly, to a process in which fluorine is produced by dissolving CaF.sub.2 in a molten salt which contains an alkali metal tetrafluoroborate and electrolyzing the melt at a temperature below 400.degree. C.
Fluorine is the most reactive of all known chemical elements, and it is prepared commercially by the electrolysis of hydrogen fluoride (HF). HF is prepared by reacting CaF.sub.2, a naturally occurring mineral known as fluorspar, with sulfuric acid. During the electrolysis, F.sub.2 is produced at the anode, and H.sub.2, which is vented and burned, is produced at the cathode. It would be desirable, therefore, to be able to produce fluorine directly from CaF.sub.2 and thereby eliminate the intermediate production of HF and the venting and burning of H.sub.2.
U.S. Pat. No. 3,684,667 describes a method for producing fluorine and volatile fluorine compounds by the electrolysis of an electrolyte consisting of molten fluorides, e.g., calcium fluoride, using a plasma as the anode. The energy required to maintain a plasma is relatively high and can be expensive.
U.S. Pat. Nos. 3,196,090 and 3,196,091 describe the electrolysis of sodium tetrafluoroborate (NaBF.sub.4) at about 440.degree. to 480.degree. C. between a lead cathode and a graphite or gold anode. A fluorine/BF.sub.3 mixture is produced at the anode, and a lead/sodium alloy is produced at the cathode. The NaBF.sub.4 starting material, however, is derived from HF as an intermediate.
Other attempts to produce fluorine directly from the electrolysis of CaF.sub.2 have been unsuccessful and have resulted instead in the production of fluorocarbons. For example, British patent No. 863,635 describes the electrolysis of CaF.sub.2 at 1450.degree. C. using a carbon anode to produce CF.sub.4 ; British patent No. 863,602 describes the electrolysis of a mixture consisting of NaF, CaF.sub.2, and MgF.sub.2 to produce C.sub.1 to C.sub.3 fluorocarbons; and U.S. Pat. No. 2,835,711 describes the electrolysis of carbon with CaF.sub.2 and group IIA metal chlorides at temperatures of from 2000.degree. to 5000.degree. C. to produce fluorocarbons and chlorofluorocarbons.